The cephalosporin derivatives which have a 2-(2-aminothiazole-4-yl)-2-methoxyiminoacetoamido group at the 7-position are known as the antibiotics having a strong antibacterial activity.
For instance, (6R,7R)-7-[(Z)-2-(2-aminothiazole-4-yl)-2-methoxyiminoacetoamido]-3-(1-qui nuclidiniummethyl)-3-cephem-4-carboxylate which is expressed by the following formula ##STR1## has a betaine structure and this compound is a cephalosporin derivative which has a strong antibacterial activity against Gram-positive and Gram-negative bacterials (Japanese Laid-Open Patent Publication No. 219292/1984), corresponding to U.S. Pat. No. 4,525,473.
Generally speaking, however, these cephalosporin derivatives are chemically unstable since the .beta.-lactam ring in their molecules is liable to be hydrolyzed.
It is, therefore, essentially important to have such cephalosporin derivatives prepared in a stable form when they are to be used as pharmaceuticals.